Understanding iOS 6 Background App Refresh Issues

Introduction to Background App Refresh on iOS 6

Background App Refresh (BAR) is a feature introduced by Apple in iOS 4, allowing apps to run background tasks even when the user has left the app or switched to another app. This feature was further enhanced in iOS 5 and became even more powerful with the release of iOS 6.

However, this feature also introduces complexities that can lead to issues if not handled correctly. In this article, we will delve into one such issue where an app crashes after returning from background on iOS 6.

Background App Refresh on iOS 6

In iOS 6, the system no longer requires apps to explicitly request permission to run in the background when activated by the user. Instead, it is now enabled by default for all apps that were previously supported by the feature.

However, this change introduces a new requirement: if an app is running in the background and needs to send messages or perform other network operations, it must ensure that these operations are properly synchronized with the system’s notifications mechanism.

The Problem

The problem described in the Stack Overflow question occurs when an app returns from the background after being inactive for some time. In this case, the app is supposed to resume its previous state by calling WillEnterForeground(), but instead, it crashes about 10 seconds later.

To understand what might be causing this issue, let’s analyze the console output provided in the question.

Console Output Analysis

The console output shows several warnings and errors related to the system trying to resume an app that has crashed or become unresponsive. Specifically, we see:

• TrekkTrakker failed to resume in time - This warning suggests that the app was not able to resume correctly when activated by the system.
• Forcing crash report of TrekTrakkerSimp[985]... - This line indicates that the system had to force a crash report for the app, which is an indication of a severe error.
• Application 'UIKitApplication:TrekkTrakker[0x28c2]' exited abnormally with signal 9: Killed: 9 - This line shows that the app was terminated abruptly due to a fatal error.

Possible Causes

Given the console output, there are several possible causes for this issue:

• The app is not properly synchronizing its background operations with the system’s notifications mechanism.
• There is an object or variable that is not retained or which is thought to be but isn’t, causing a crash when trying to access it.

Resolving the Issue

To resolve this issue, we need to carefully examine our code and ensure that all objects and variables are properly retained and synchronized with the system’s notifications mechanism.

Here are some general steps we can follow:

1. Review your app’s background operations: Make sure that you are synchronizing your background operations correctly with the system’s notifications mechanism.
2. Check for retain issues: Ensure that any objects or variables used in background operations are properly retained and not deallocated too quickly.
3. Run in the debugger: Use the Xcode debugger to examine the code at the point where the crash occurs and look for any potential issues.

Code Example

Here is a simplified example of how we might implement proper synchronization with the system’s notifications mechanism:

{
  <highlight language="Objective-C">
}
// In our view controller implementation file
- (void)viewDidLoad {
    [super viewDidLoad];
    
    // Check if background refresh is enabled
    if ([UIApplication sharedApplication].backgroundRefreshEnabled) {
        // Register for background refresh
        [[UIApplication sharedApplication] registerForBackgroundTasks];
        
        // Perform any necessary background operations here...
    }
}

- (void)applicationWillEnterForeground:(NSNotification *)notification {
    // Handle foreground activation...
}

- (void)applicationDidBecomeActive:(NSNotification *)notification {
    // Perform any necessary updates to the app's state here...
}
{
  </highlight>
}

In this example, we first check if background refresh is enabled and register for it using registerForBackgroundTasks. We then perform our necessary background operations. In the applicationWillEnterForeground method, we handle foreground activation by performing any necessary updates to the app’s state.

By following these steps and being mindful of the potential issues that can occur with background app refresh on iOS 6, we can ensure that our apps are properly synchronized with the system’s notifications mechanism and avoid crashes when returning from background.

Last modified on 2024-06-12